Mesoporous Materials
A material whose molecular structures allow fluid flow through the material is porous. Porous materials can be characterized by their pore sizes. Very small pores having diameters less than 2 nanometers (nm) are called micropores, while very large pores having diameters greater than 50 nm are called macropores. Microporous materials offer a large surface area per volume of material but the micropores are too small to be readily accessible to liquids. In contrast, macropores are large enough to afford ready access to liquids but afford smaller surface areas relative to equivalent volumes of smaller pore materials. Pores of intermediate size—i.e., those having diameters between 2 and 50 nm—are called mesopores and form the subject of the present invention. One aspect of mesoporous materials is that they offer greater liquid access than micropores and greater surface area per unit volume than macropores. Consequently, mesoporous materials provide liquid access to more surface area per unit volume of material than either microporous or macroporous materials. As used herein, the term “mesopore” will be used to refer to pores in the desired size range, namely to pores having diameters between approximately 2 and 50 nm.
Uses for Mesoporous Materials
Because of their large liquid-accessible surface areas, mesoporous materials are useful in many liquid phase applications, including as sorbents, electrical materials and catalyst and chromatographic supports. In the field of catalysis, reactions typically occur at surfaces—i.e., the surface of the catalyst serves as an active site for the combination or separation of reactive species and the available surface area therefore limits reaction rates. Catalytic reactions can occur in the gas or liquid phase; in either case, it is desirable to maximize the amount of catalyst surface area; the larger the surface area of catalyst, the greater the number of active sites and the faster the rate of the reaction. One common way to increase surface area is to provide the catalyst as a thin coating on the surface of a high surface area support material. The support material provides the structure for the catalyst and thus determines its shape and the amount of surface area per unit volume. Hence, the porosity and surface area of the support become rate-limiting factors for the catalytic reaction. Mesoporous materials provide optimal support structures for certain catalytic systems because they provide both high surface area and liquid accessibility.
Another application in which a mesoporous polymer is advantageous is in high performance liquid chromatography (HPLC), a technique commonly used for separating and quantifying the constituents of a mixture. HPLC is often used to separate chemicals and biological molecules that have very similar properties and are difficult if not impossible to separate by other conventional means. One fundamental limitation of HPLC is that, because the columns are packed with small porous beads, the high flow rates required to maximize the throughput in preparatory scale separations result in channeling of the solution around the particles, rather than through the pores. This, in turn, impedes the separation. Because separation is a major cost of chemical processing, the development of high capacity monolithic columns could greatly reduce the cost of manufacturing pharmaceuticals and their precursors. The mesoporous polymers described herein may be prepared by polymerization within a suitable structure (e.g., a glass or metal tube) to form a stationary phase for chromatography.
Pore size is also an important aspect of sorbent technology. This technology involves the sorption of a sorbate in the liquid or gas phase into a solid phase sorbent. The liquid phase can comprise either a solvent containing a dissolved sorbate or an emulsion or other mixture of two liquids, one of which is the sorbate. Examples of common sorbents currently include polymers and carbons in powdered, granular, and pelletized form. These are used in environmental applications associated with energy production, by-product recoveries, and waste incineration, as well as water purification and wastewater cleanup. An example of a sorbent application that involves a gas phase is air purification. A variety of other uses are known or are being developed. For each desired application, the sorbent is selected such that the sorbent has an affinity for the sorbate species, which is attracted into the solid and held there by one of various surface mechanisms. The efficacy of a sorbent material depends on how much sorbate it can attract and retain. Hence, the pore size and available surface area are critical in this context as well.
Certain electrical applications also involve liquid phase, surface-limited reactions. One example of such an application is an ultracapacitor. Like batteries, ultracapacitors are energy storage devices. Ultracapacitors are notable for their ability to store and deliver energy at high power densities, and to be cycled virtually indefinitely without degradation. In contrast, batteries store large amounts of energy, but function most efficiently at low power densities and degrade quickly if they are repeatedly cycled. The characteristics of ultracapacitors make them particularly suitable to meet the power requirements of various emerging technologies, including electric vehicles, electronics (cellular telephones, and digital communications) and clean power (uninterrupted power sources, filters, etc.).
An ultracapacitor typically comprises a pair of electrodes separated by a non-conductive porous separator. The space between the electrodes is filled with a liquid electrolyte, which can be either aqueous or organic. Because no chemical reactions occur during the charge/discharge cycle, capacitors can be cycled many times without degradation, unlike batteries. Unfortunately, though, conventional ultracapacitors lacked sufficient energy storage capacity to be commercially practical. One key to improving the energy storage capacity of ultracapacitors is to optimize the interaction between the electrodes and the electrolyte.
There are two major categories of electrolytes for double layer devices: aqueous and organic, each of which has advantages and disadvantages. Aqueous electrolytes such as potassium hydroxide and sulfuric acid offer low electrical resistance (0.2 to 0.5 ohms/cm2) and can therefore be charged and discharged very quickly. However, they can only be cycled through a potential range of approximately one volt due to the generally low breakdown voltages of aqueous electrolytes; this sharply limits their energy storage density, which is proportional to the square of the voltage. Organic electrolytes such as propylene carbonate offer breakdown voltages of up to three volts and therefore have energy storage densities as much as nine times higher than those of aqueous electrolytes. However, due to their much higher electrical resistance of 1–2 ohms/cm2, organic electrolytes reduce the speed with which capacitors can be cycled. The type of electrolyte that is desirable depends on the nature of the application.
The mechanism for energy storage devices of this type is based on the double-layer capacitance at a solid/liquid interface. More specifically, double-layer ultracapacitors typically consist of high surface area carbon structures that store energy in a polarized liquid layer. The polarized liquid layer forms at the interface between an ionically conducting liquid electrolyte and an electronically conducting electrode, namely the carbon structure. As illustrated in FIG. 1, the separation of charge in the ionic species at the interface (called a double layer) produces a standing electric field. Thus, the capacitive layer, while only a few angstroms thick, has a very large area. The larger the area of the solid/liquid interface, the more energy can be stored. Hence, the capacitance of this type of capacitor is proportional to the surface area of the electrode.
At the same time, electrodes having pores smaller than about 2 nm do not exhibit increased capacitance. It is believed that pores smaller than about 2 nm are too small to allow entry of most nonaqueous electrolytes and therefore cannot be fully wetted, with the result that a portion of the potential interface area is not realized. Hence, it is believed that mesoporous materials are optimal for use in this type of capacitor.
While some carbon structures having pore sizes in the mesoporous range have been extensively investigated for use in ultracapacitors because of their low cost and potential for high-energy storage densities, none of them has proved entirely satisfactory. Since the capacitance of the material increases linearly with the specific surface area, a carbon material with a capacitance of 20 μF/cm2 and a surface area of 1000 m2/g would have a capacitance of 200 F/g if all of the surface area were electrochemically accessible. However, since high surface area porous carbons typically have a high fraction of micropores, only a fraction of the surface of the carbon is effectively utilized—i.e., wetted. Therefore, most of the electrode surface area does not contribute to the double-layer capacitance and the measured capacitance values of prior carbon structures are therefore only about 20 percent of theoretical. For carbon-based ultracapacitors to approach their theoretical performance, they should have a high pore volume (>50%) and a high fraction of continuous pores with diameters greater than 2 nm to allow the electrolyte access to the carbon surface.
In sum, the major drawbacks of the carbons now used in double-layer ultracapacitors are: low capacitance (due to pores that are too large to offer high surface area or too small to offer liquid access), and high costs (due to materials and processing costs).
Manufacture of Mesoporous Carbon-Based Materials
One well understood polymeric system is the resorcinol/formaldehyde (“RF”) system (see FIG. 2), which is a member of the hydroxylated benzene/aldehyde polymer family. During the polymerization, resorcinol serves as a trifunctional monomer capable of adding formaldehyde in the 2, 4, and 6 positions of the resorcinol aromatic ring. The resorcinol monomer is particularly reactive because of the electron-donating effects of the attached hydroxyl groups. In solution, the substituted resorcinol rings condense with each other to form clusters. In previous work, an alkaline salt such as Na2CO3 or K2CO3 is added to catalyze this reaction and it has been shown that the size of the polymer clusters is regulated by the catalyst concentration in the RF solution (Tamon et al. 1998). Increasing basicity results in smaller polymer clusters. The catalyst concentration is typically listed in terms of R/C, which is the molar concentration of resorcinol, or its functional equivalent, divided by the molar concentration of catalyst.
In order to introduce larger pores into polymers, and thus increase their porosity, several groups have prepared polymeric gels, such as RF gels, in liquid emulsions. The term “gel” means only that a process such as polymerization has occured in a liquid mixture causing the mixture to become more viscous or, in some cases, to become a solid. Such RF gels usually have a mixture of pore sizes, and both the polymers and the carbons formed therefrom by pyrolysis are mostly macroporous (i.e., having diameters greater than 50 nm) rather than mesoporous (LeMay et al. 1990, Even and Gregory 1994). An alternative approach uses aerogels, which are gels dried by supercritical fluid processing. Supercritical fluid processing is known to minimize shrinkage and pore collapse on drying, but is a relatively expensive processing technique.
It is well known that many polymers, including RF polymers and other thermoset polymers, can be pyrolyzed to yield carbon materials. The term “carbon materials” means materials that consist primarily of the element carbon and have properties associated with graphite, such as a useful level of electrical conductivity. Pyrolysis of polymer aerogels prepared by supercritical fluid extraction of RF gels produces carbons with a mixture of meso- and micropores (Pekala et al. 1994). However, because of the need for supercritical extraction, these carbon aerogels are very expensive to make. Hence, an effective method for producing a mesoporous carbon that does not involve supercritical fluid extraction is particularly desirable.
In U.S. Pat. No. 6,297,293, we described a novel method for producing mesoporous carbon using surfactants and a carbon-based polymer system without the need for supercritical fluid extraction. In this method, surfactant micelles serve as a template around which polymerization of carbon-based pre-polymers takes place. Following polymerization, the surfactant is removed, leaving pores whose size depends on the physical dimensions of the micelles. Although this technique allows control of pore size through proper surfactant selection, a significant drawback is the inherent requirement for surfactants, which are relatively expensive and therefore add significantly to the overall cost.
RF gels prepared at higher catalyst concentrations are transparent. It is also known that at high catalyst concentrations (e.g. R/C=50), the RF polymerization produces polymers that upon drying (either conventional or supercritical) have a large proportion of micropores. This observation has been attributed to initially very small polymer structure consisting of an assemblage of roughly spherical particles. The space between these particles is related to the size of the particles. Just as a stack of bowling balls would give larger crevices than a stack of golf balls, so too would a mass of larger polymer particles be expected to yield larger spaces between particles. The spaces between particles act as pores that allow the entrance of fluids into the polymer. Other factors being equal, polymers consisting of larger particles should have larger pores, and upon pyrolysis should yield carbons with larger pores.
The catalyst concentration also determines the pH of the reaction mixture and transparent gels are prepared from solutions having a pH between 6.5 and 7.4. These transparent gels shrink and crack upon simple evaporative drying and therefore are typically dried by supercritical extraction with carbon dioxide.
In contrast, it is known that RF gels prepared at low catalyst concentration (R/C>900) are opaque due to the formation of large particles and pore spaces. In a patent by Droege (1999) it was shown that gels prepared with R/C of approximately 1000 and a pH of approximately 6 produces opaque gels that can withstand simple evaporative drying and the carbon derived from carbonizing this polymer has some mesopores. Tamon et al. (1998) showed that by adjusting R/C and the amount of water they could make mesoporous carbons, but their methods can only prepare carbons with mesopores in the range of 2 to 6 nm. (Tamon et al. 1998). For a general discussion on resorcinol, see Dressler, H. (1994). Both references are hereby incorporated by reference.
In view of this, and the known drawbacks of the prior art, it is desired to provide an improved mesoporous polymer and carbon structure requiring neither supercritical fluid extraction nor surfactants. The polymer structure should be simple and inexpensive to manufacture and should have a high pore volume and a high fraction of mesopores. When intended for use as electrodes for ultracapacitors, the desired carbon structure should have high gravimetric and volumetric capacitance.